U.S. Pat. Nos. 4,360,488 and 4,385,026 disclose formation of "non-draining" gels by heating PTFE with a highly fluorinated high-boiling material at a temperature close to the crystalline melting point of the polymer (330-350.degree. C.). A solution or swollen mass containing from about 1 to about 50 weight % polymer is formed on heating, from which is recovered, on cooling, a sponge-like gel, said gel being without defined shape and retaining no "memory" of the crystallinity of the original PTFE. The gel, after removal of the fluorinated material by extraction in refluxing solvent such as Freon.RTM.-113 (bp 45.8.degree. C.), is described as porous, and could be formed into porous shapes, e.g., into porous sheet by pressing between platens. The process appears to employ granular PTFE only and, because crystalline memory is lost during processing, the initial gels are shapeless globs which require post fabrication into shaped articles. The porous products have increased crystallinity and a partially fibrillar structure. Use as filter membranes or diaphragms for electrochemical cells is disclosed.
Microporous PTFE films and sheeting are known. U.S. Pat. No. 3,664,915 discloses uniaxially stretched film having at least 40% voids and a highly fibrillar structure. U.S. Pat. Nos. 3,953,566, 3,962,153 and 4,187,390 disclose porous PTFE films having at least 70% voids, said films consisting of nodes and fibrils wherein the nodes are at least 1000 times thicker than the fibrils. Pore size in the above films is at least 0.2 .mu.m. Unsintered, paste-extruded PTFE film is stretched at rates of over 2000%/sec and as high as 40,000%/sec to achieve porosity, followed by sintering nder constraint at 327 to 370.degree. C. Such stretching rates are far higher than those employed in conventional film preparation. U.S. Pat. No. 4,110,392 discloses microporous PTFE films having pore sizes as low as 0.01 .mu.m, achieved by stretching unsintered PTFE as above, followed by sintering without constraint, then stretching a second time at high speed. Porosities of these films are 10 to 50%. Japanese Application 3-221541 discloses microporous PTFE film with pore size of 0.1 to 0.2 .mu.m. The films are prepared by a modification of the above-described art procedures and have different film morphology and improved air permeability. However, ultrahigh stretching rates are again required. The art does not provide the means of improving porous PTFE articles by stretching at conventional rates.
U.S. Pat. No. 5,110,527 discloses porous PTFE films exhibiting pore sizes of at least 15 .mu.m and voids of 80% and greater fabricated by a method in which a blend of PTFE resins of high and low molecular weight is paste-extruded into a sheet, stretched at temperatures below the melting point, and finally sintered while held in the stretched state by exposure to temperatures of at least 327.degree. C.